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19-0226 Rev 0 1 / 94

Transformer Driver for Isolated RS-485 Interface

19-0226 Rev 0 1 / 94
Transformer Driver for Isolated RS-485 Interface
o Power-Supply Transformer Driver for Isolated RS-485 / RS-232 Data-Interface Applications o Single +5V or +3.3V Supply o Low-Current Shutdown Mode: 0.4µA o Pin-Selectable Frequency: 350kHz or 200kHz o 8-Pin DIP, SO, and µMAX Packages
MAX253
PART MAX253CPA MAX253CSA MAX253CUA MAX253C / D MAX253EPA MAX253ESA MAX253EUA MAX253MJA TEMP. RANGE 0°C to +70°C 0°C to +70°C 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C -40°C to +85°C -55°C to +125°C PIN-PACKAGE 8 Plastic DIP 8 SO 8 µMAX Dice 8 Plastic DIP 8 SO 8 µMAX 8 CERDIP
Isolated RS-485 / RS-232 Power-Supply Transformer Driver High Noise-Immunity Communications Interface Isolated and / or High-Voltage Power Supplies Bridge Ground Differentials Medical Equipment Process Control
Contact factory for dice specifications. Contact factory for availability and processing to MIL-STD-883.
ON / OFF 4 SD 6 VCC D1 1 C3 C2 VIN 5V C1 OUTPUT 5V @ 200mA
TOP VIEW
D1 GND1
D2 GND2 VCC N.C.
MAX253
3 FREQUENCY SWITCH FS GND1 2 D2 GND2 7 8
MAX253
DIP / SO / µMAX
Call toll free 1-800-998-8800 for free samples or literature.
Transformer Driver for Isolated RS-485 Interface MAX253
ABSOLUTE MAXIMUM RATINGS
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
Note 1: Operating supply current is the current used by the MAX253 only, not including load current. Note 2: Shutdown supply current includes output switch-leakage currents.
Transformer Driver for Isolated RS-485 Interface
plot01
MAX253
plot02
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
INCLUDES SWITCH LEAKAGE CURRENTS SHUTDOWN CURRENT (µA) 0.8
plot03
15 MEASURED AT TP1 OUTPUT RESISTANCE ()
6 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
plot04
plot05
plot06
240 FREQUENCY (kHz)
160 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
-60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
plot07
plot08
-60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
200 20 40 60 80 100120 140 160 180 LOAD CURRENT (mA)
Transformer Driver for Isolated RS-485 Interface MAX253
plot09
plot10
plot11
MEASURED AT TP1 0 20 40 60 80 100 120 140 160 180 200 220 LOAD CURRENT (mA)
SWITCHING WAVEFORMS (TWO CYCLES)
SWITCHING WAVEFORMS (BREAK BEFORE MAKE)
CIRCUIT OF FIGURE 1
TIME FROM SHUTDOWN TO POWER-UP
TP1 (OUTPUT VOLTAGE)
CIRCUIT OF FIGURE 6
Transformer Driver for Isolated RS-485 Interface
MAX253
VIN 5V C1 0.1µF 4 ON / OFF
MAX253
3 FREQUENCY SWITCH FS GND1 2 GND2 7
Figure 1. Test Circuit
VIN C1 F / F VCC Q T OSC FREQUENCY SWITCH 400kHz / 700kHz Q
5V @ 200mA ISO OUTPUT C2
MAX253
D2 N SD GND2 GND1 ISO GND
Figure 2. Block Diagram
quency (see Figure 2). These two signals drive the ground-referenced output switches. Internal delays ensure break-before-make action between the two switches. Ground SD for normal operation. When high, SD disables all internal circuitry, including the oscillator and both power switches. Pulling FS low reduces the oscillator frequency and lowers the supply current (see Supply Current vs. Temperature in the Typical Operating Characteristics). FS includes a weak pull-up, so it will float to the high-frequency state if not connected.
Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V C1 0.1µF
ISOLATION BARRIER
ICT:1.3CT 1N5817 C3 0.1µF C2 22µF
ISO 5V C4 22µF
MAX253
D2 8 3 1N5817
MAX667
GND1 2
GND2 7
SHDN 5
PC410 / 417
3.3k 6
74HC04
MAX481 MAX483 MAX485 MAX487
74HC04
74HC04 OR EQUIVALENT SEE TABLE 2
Figure 3. Typical RS-485 Application Circuit, 5V Configuration
Transformer Driver for Isolated RS-485 Interface MAX253
VIN 3.3V C1 0.1µF 5 4 6 1
ISOLATION BARRIER
ICT:2.1CT 1N5817 C3 0.1µF C2 22µF
N.C. SD VCC GND1 2 GND2 7
ISO 5V C4 22µF
MAX253
D2 FS 8 3 1N5817
MAX667
SHDN 5
1N5817
C5 0.1µF PC410 / 417
74HC04
4 PC357T 4 4 DI
74HC04
MAX481 MAX483 MAX485 MAX487
74HC04
74HC04 OR EQUIVALENT SEE TABLE 2
Figure 4. Typical RS-485 Application Circuit, 3.3V Configuration
Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V C1 0.1µF 5 6 N.C. VCC D1 1 ISOLATION BARRIER ICT:1.3CT1N5817 C3 0.1µF 8 3 1N5817 5 x 3.3k 10 x PC417 74HC04 T1IN 74HC04 T2IN 74HC04 T3IN 74HC04 T4IN 74HC04 T5IN 5 X 3.3k 74HC04 R1OUT 74HC04 R2OUT 74HC04 R3OUT 74HC04 R4OUT 74HC04 R5OUT 74HC04 OR EQUIVALENT 390 14 390 17 390 23 6 5 4 1 2 390 6 R2OUT R2IN 5 390 9 390 22 390 16 T4IN T4OUT 1 390 15 390 390 1 2 6 5 4 7 8 VCC T1IN GND 3 T1OUT SET GND SHDN 6 4 5 8 2 IN OUT C2 22µF MAX667 ISO 5V C4 22µF
MAX253
FS GND1 GND2 2 7
T2OUT
T3OUT
T5OUT
MAX205
R1OUT R1IN 10
R3OUT
R4OUT
R5OUT SD 21
R5IN EN 20
SEE TABLE 2
4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps) VCC 1N5711 4N25 6 1N5711 6 4N25 3.3k 3.3k 390 1 1 TIN ISO ROUT 5 5 TIN 390 74HCO4 74HC04 2 2 ISO ISO 4 4 GND GND
VCC ISO ROUT
Figure 5. Typical RS-232 Application Circuit
Transformer Driver for Isolated RS-485 Interface
Figures 3-5 are typical isolated RS-485 / RS-232 data-interface circuits. These circuits withstand 1800VRMS (1sec) and are intended for industrial communications and control applications where very high voltage transients, differential ground potentials, or high noise may be encountered. Table 2 lists transformer characteristics for the applications of Figures 3-10. Some suggested manufacturers of transformers, transformer cores, and optocouplers are listed in Table 3, along with their respective phone and fax numbers. Important layout considerations include: o For maximum isolation, the "isolation barrier" should not be breached. Connections and components from one side should not be located near those of the other side. o Since the optocoupler outputs are relatively highimpedance nodes, they should be located as close as possible to the Maxim interface IC. This minimizes stray capacitance and maximizes data rate. Refer to the µMAX package information for pin spacing and physical dimensions. appropriate Maxim interface device for data-transfer rates up to 2.5Mbps. Refer to the MAX1480 data sheet for a complete isolated RS-485 solution in one package.
MAX253
Isolated RS-232 Data Interface
The MAX253 is ideal for isolated RS-232 data-interface applications requiring more than four transceivers. The 1W power output capability of the MAX253 enables it to drive more than 10 transceivers simultaneously. Figure 5 shows the typical application circuit for a complete 120kbps isolated RS-232 data interface. The figure also shows how the Sharp PC417 optocouplers can be replaced by the lower-cost 4N25 devices to achieve data-transfer rates up to 9.6kbps. For 3.3V operation, substitute the primary portion of Figure 5 with the circuit of Figure 7. For applications requiring two transceivers or fewer, refer to the MAX250 / MAX251 or MAX252 data sheet.
Isolated Power Supplies
The MAX253 is a versatile isolated power driver, capable of driving a center-tapped transformer primary from a 5V or a 3.3V DC power supply (see Figures 6 and 7). The secondary can be wound to provide any isolated voltage needed at power levels up to 1W with a 5V supply, or 600mW with a 3.3V supply. Figure 6 shows a typical 5V to isolated 5V application circuit that delivers up to 200mA of isolated 5V power. In Figure 7, the MAX253 is configured to operate from a 3.3V supply, deriving a "boost" VCC for the MAX253 by connecting diodes to both ends of the transformer primary. This produces nearly double the input supply, and may be useful for other applications, as shown in Figure 4. The average current in each MAX253 switch must still be limited to less than 200mA, so the total power available is approximately 600mW.
Isolated RS-485 Data Interface
The MAX253 power-supply transformer driver is designed specifically for isolated RS-485 data-interface applications. The application circuits of Figures 3 and 4 combine the MAX253 with a low-dropout linear regulator, a transformer, several high-speed optocouplers, and a Maxim RS-485 interface device. With a few modifications to these circuits, full-duplex communications can be implemented by substituting the MAX481 / MAX485 with the MAX490 / MAX491 (for data rates up to 2.5Mbps) or substituting the MAX483 / MAX487 with the MAX488 / MAX489 (for data rates up to 250kbps). The data transfer rates of the application circuits in Figures 3 and 4 are critically limited by the optocouplers. Table 1 lists suggested optocouplers and the
Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates
DATA RATE 250kbps 2.5Mbps FULL DUPLEX RS-485 IC MAX488 / MAX489 MAX490 / MAX491 HALF DUPLEX RS-485 IC MAX483 / MAX487 MAX481 / MAX485 OPTOCOUPLER FOR DI / RO PC417 PC410 OPTOCOUPLER FOR DE PC357T PC357T
Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V C1 0.1µF 4 ON / OFF
6 VCC SD D1 1 ICT:1.3CT1N5817 C3 0.1µF TP1 C2 22µF 5V @ 200mA ISO OUTPUT
MAX253
3 FREQUENCY SWITCH FS GND1 2 GND2 7
OUTPUT
8 1N5817
OPTIONAL 21kHz LOWPASS OUTPUT FILTER L2 25µH FILTER OUTPUT C7 2.2µF
SEE TABLE 2
Figure 6. 5V to Isolated 5V Application Circuit
VIN 3.3V C1 0.1µF 4 ON / OFF 1 ICT:2.1CT 1N5817 C3 0.1µF 5V @ 100mA TP1 ISO OUTPUT C2 22µF
MAX253
3 FREQUENCY SWITCH FS GND1 2 GND2 7 D2 VCC 6 1N5817 1N5817
OUTPUT
8 1N5817
OPTIONAL 21kHz LOWPASS OUTPUT FILTER L2 25µH FILTER OUTPUT C7 2.2µF
SEE TABLE 2
C4 0.1µF
Figure 7. 3.3V to Isolated 5V Application Circuit
Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V 6 VCC D1 1 ISOLATION BARRIER 1CT:5CT 1N5817 24V UNREGULATED 10µF
MAX253
4 SD GND1 2 GND2 7 D2 8 1N5817 78L05
5V 0.1V to 0.5V
IL300
MAX480
ISO 5V 3 49.9k 2 6
4 49.9k
MAX480
2N3904 2N3904 10k
SEE TABLE 2
Figure 8. Typical 4mA to 20mA Application Circuit
Output-Ripple Filtering A simple lowpass pi-filter (Figures 6 and 7) can be added to the output to reduce output ripple noise to about 10mVp-p. The cutoff frequency shown is 21kHz. Since the filter inductor is in series with the circuit output, minimize its resistance so the voltage drop across it is not excessive.
Isolated 4mA to 20mA Analog Interface
The 4mA to 20mA current loop is a standard analog signal range that is widely used in the process-control industry for transducer and actuator control signals. These signals are commonly referred to a distant ground that may be at a considerably higher voltage with respect to the local ground. An analog signal in the range of 0.1V to 0.5V is applied to the first MAX480 to generate a signal current in the range of 20µA to 100µA. This low-level signal is transferred across the barrier by the Siemens IL300 linear optocoupler. This device is unique in that it corrects the dominant nonlinearity present in most optocou-
plers-the LED efficiency variation. The IL300 is really two optocouplers in the same package sharing the same LED one detector is across the isolation barrier, the other is on the same side as the LED (Figure 8). The latter detector is used to generate a feedback signal identical to the signal on the isolated side of the barrier. The current signal transferred across the barrier is converted back to a voltage that matches the input in the 100mV to 500mV range. This voltage is then transformed to the final 4mA to 20mA current signal range by the second MAX480, Darlington stage, and the 20 resistor.
Isolated ADC
Almost any serial-interface device is a candidate for operation across an isolation barrier Figure 10 illustrates one example. The MAX176 analog-to-digital converter (ADC) operates from +5V and -12V supplies, provided by the multiple-tapped secondary and linear regulators. If some additional isolated power is needed for signal conditioning, multiplexing, or possibly for a
Transformer Driver for Isolated RS-485 Interface MAX253
VIN INPUT 6 VCC D1 1 1CT:1CT 1N5817 +VOUT 2V IN OUTPUT RL+ 8 RL+ RLRL-VOUT -2V IN OUTPUT
MAX253
D2 GND2 7
GND1 2
SEE TABLE 2
1N5817
Figure 9a. Half-Wave Rectifier-Bipolar
VIN INPUT
6 VCC D1 1 1CT:1CT 4 x 1N5817
MAX253
D2 GND1 2 SEE TABLE 2 GND2 7 8
VOUT +VIN OUTPUT
VOUT -VIN OUTPUT
Figure 9b. Full-Wave Rectifier-Bipolar
VIN INPUT
6 VCC D1 1 1CT:1CT 4 x 1N5817
MAX253
D2 GND1 2 GND2 7 8
VOUT 2 x VIN OUTPUT
SEE TABLE 2
Figure 9c. Full-Wave Rectifier-Unipolar
Transformer Driver for Isolated RS-485 Interface MAX253
ISOLATION BARRIER 1CT : 1.5CT : 3CT 78L05 ISO 5V 10µF 4 x 1N5817
VIN 5V
1 6 8 79 L12 ISO -12V 10µF +5V
D1 VCC D2 GND1 2 GND2 7
MAX253
74HC04
START INPUT CLOCK
8 7 3k 6 10µF 5 VSS CONVST CLOCK DATA 8 8 7 6 5 3k 6 470 5 1 2 3 SIGNAL GROUND 4 7
6N136
74HC595 QF
MAX176
0.1µF ANALOG INPUT 1 2 3 4 GND VDD AIN VREF
6N136
D11(MSB) D10 D9 D8 +5V 0.1µF
0.1µF 10µF
74HC04
6N136
0.1µF 10µF
8 7 6 5 8.2k 14 11 12 10 SER SCK RCK SCLR 8 QH
D7 D6 D5 D4 D3 D2 D1 D0(LSB) +5V 0.1µF
74HC595 QF
SEE TABLE 2
Figure 10. Typical Isolated ADC Application
Transformer Driver for Isolated RS-485 Interface MAX253
sensor, an extra several hundred milliwatts could easily be supplied by the circuit, as shown. A +12V supply could be generated by adding two more diodes to the ends of the secondary, and a -5V supply could be generated by connecting additional diodes to the 1 / 4 and 3 / 4 tap points on the secondary. For +5V only applications, the MAX187 is recommended. for half the primary is simply the product of the maximum supply voltage and half the maximum period. With FS tied high, the guaranteed minimum frequency is 250kHz, giving a maximum period of 4µs. The secondary winding may or may not be center tapped, depending on the rectifier topology used. The phasing of the secondary winding is not critical. In some applications, multiple secondaries might be required. Half-wave rectification could be used, but is discouraged because it normally adds a DC imbalance to the magnetic flux in the core, reducing the ET product. If the DC load is imbalanced, full-wave rectification is recommended, as shown in Figure 9b. The transformer turns ratio must be set to provide the minimum required output voltage at the maximum anticipated load with the minimum expected input volt-
Transformer Selection
Table 2. Typical Transformer Characteristics
Table 3. Transformer, Transformer Core, and Optocoupler Suppliers
TRANSFORMERS BH Electronics Phone: (507) 532-3211 FAX: (507) 532-3705 Coilcraft Phone: (708) 639-6400 FAX: (708) 639-1469 Coiltronics Phone: (407) 241-7876 FAX: (407) 241-9339 TRANSFORMER CORES Philips Components Phone: (407) 881-3200 FAX: (407) 881-3300 Magnetics Inc. Phone: (412) 282-8282 FAX: (412) 282-6955 Fair-Rite Products Phone: (914) 895-2055 FAX: (914) 895-2629 OPTOCOUPLERS Quality Technology Phone: (408) 720-1440 FAX: (408) 720-0848 Sharp Electronics Phone: (206) 834-2500 FAX: (206) 834-8903 Siemens Components Phone: (408) 777-4500 FAX: (408) 777-4983
Transformer Driver for Isolated RS-485 Interface
age. In addition, include in the calculations an allowance for worst-case losses in the rectifiers. Since the turns ratio determined in this manner will ordinarily produce a much higher voltage at the secondary under conditions of high input voltage and / or light loading, be careful to prevent an overvoltage condition from occurring (see Output Voltage vs. Load Current in the Typical Operating Characteristics). Transformers used with the MAX253 will ordinarily be wound on high-permeability magnetic material. To minimize radiated noise, use common closed-magneticpath physical shapes (e.g., pot cores, toroids, E / I / U cores). A typical core is the Philips 213CT050-3B7, which is a toroid 0.190" in diameter and 0.05" thick. For operation with this core at 5.5V maximum supply voltage, the primary should have about 22 turns on each side of the center tap, or 44 turns total. This will result in a nominal primary inductance of about 832µH. The secondary can be scaled to produce the required DC output. is a good choice for through-hole applications, and the NIEC SB05W05C dual in an SOT-23 package is recommended for surface-mount applications. Use the higher frequency setting to reduce ripple.
MAX253
Diode Selection
Table 4. Suggested Capacitor Suppliers
PRODUCTION METHOD CAPACITORS Matsuo 267 series (low ESR) USA Phone: (714) 969-2491, FAX: (714) 960-6492 Sprague Electric Co. 595D / 293D series (very low ESR) USA Phone: (603) 224-1961, FAX: (603) 224-1430 Murata Erie Ceramic USA Phone: (800) 831-9172, FAX: (404) 436-3030 Sanyo OS-CON series (very low ESR) USA Phone: (619) 661-6835, FAX: (619) 661-1055 Japan Phone: 81-7-2070-1005, FAX: 81-7-2070-1174 Nichicon PL series (low ESR) USA Phone: (708) 843-7500, FAX: (708) 843-2798 Japan Phone: 81-7-5231-8461, FAX: 81-7-5256-4158
Surface Mount
High-Performance Through Hole
Through Hole
Transformer Driver for Isolated RS-485 Interface MAX253
0.085" (2.159mm) GND1
FS SD 0.058" (1.4732mm)
TRANSISTOR COUNT: 31 SUBSTRATE CONNECTED TO VCC.
DIM A A1 B C D E e H L INCHES MAX MIN 0.036 0.032 0.008 0.004 0.014 0.010 0.007 0.005 0.120 0.116 0.120 0.116 0.0256 0.198 0.188 0.026 0.016 6° 0° MILLIMETERS MIN MAX 0.81 0.91 0.10 0.20 0.25 0.46 0.13 0.18 2.95 3.04 2.95 3.04 0.65 4.90 -- 0.55 -- 0° 6°
C A .127mm .004 in B A1 L
8-PIN µMAX PACKAGE
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.